The present invention relates to membrane electrode assemblies and to fuel cells with improved lifetime, which comprise at least two electrochemically active electrodes separated by a polymer electrolyte membrane.
Polymer electrolyte membranes (PEMs) are already known. The proton-conducting membranes used therein are at present almost exclusively sulfonic acid-modified polymers. Predominantly perfluorinated polymers are employed. A prominent example thereof is Nafion™ from DuPont de Nemours, Wilmington USA. For proton conduction, a relatively high water content in the membrane is required, which is typically 4-20 molecules of water per sulfonic acid group. The necessary water content, but also the stability of the polymer in conjunction with acidic water and the hydrogen and oxygen reaction gases, limits the operating temperature of the PEM fuel cell stack typically to 80-100° C. Under pressure, the operating temperature can be increased to >120° C. Otherwise, higher operating temperatures cannot be achieved without a loss in performance of the fuel cell.
For system reasons, however, higher operating temperatures than 100° C. in the fuel cell are desirable. The activity of the noble metal-based catalysts present in the membrane electrode assembly (MEA) is significantly better at high operating temperatures. More particularly, in the case of use of what are called reformates from hydrocarbons, considerable amounts of carbon monoxide are present in the reformer gas, which typically have to be removed by complex gas treatment or gas purification. At high operating temperatures, the tolerance of the catalysts to the CO impurities rises.
In addition, heat evolves in the operation of fuel cells. Cooling of these systems to below 80° C. can, however, be very costly and inconvenient. According to the power output, the cooling apparatuses can be made much simpler. This means that, in fuel cell systems which are operated at temperatures above 100° C., the waste heat can be utilized much better, and hence the fuel cell system efficiency can be enhanced by power-heat coupling.
In order to attain these temperatures, membranes with novel conductivity mechanisms are generally used. One approach for this purpose is the use of membranes which exhibit electrical conductivity without the use of water. The first promising development in this direction is detailed in WO 96/13872.
Since the voltage which can be tapped from a single fuel cell is relatively low, several membrane electrode assemblies are generally connected in series and are connected to one another via planar separator plates (bipolar plates). The membrane electrode assemblies and the separator plates have to be compressed with one another at comparatively high pressures in order to achieve a maximum density of the system, a maximum power and a minimum volume.
In practice, the compression of the membrane electrode assemblies with the separator plates, however, leads to problems since the polymer electrolyte membranes used have a comparatively low mechanical strength and stability and can therefore be damaged easily when compressed.
Furthermore, firstly due to the requirement for high compaction of the polymer electrolyte membrane and secondly due to the low mechanical stability thereof, it is possible only with difficulty to achieve reproducible results. Usually, the resulting fuel cell stacks have highly varying performances caused by more or less pronounced cracks in the individual membranes and/or by different degrees of compaction of the membranes. Furthermore, creeping of the electrolyte membrane is observed in long-term operation, which is attributable at least partly to the low mechanical stability.